1. Field of the Invention
This invention relates to a method of level measurement on a radar basis in which, by means of an antenna, microwaves are emitted to the surface of a material in a container and the echo waves reflected from the surface thereof are received, an actual echo function representing the echo amplitudes as a function of the distance is formed from the echo waves received by the antenna for each measurement, the probable useful echo and the transit time thereof is established from the actual echo function, and the distance of the material surface from the antenna is determined therefrom.
2. Description of the Prior Art
For radar-based level measurement all known methods may be employed which permit relatively short distances to be measured by means of reflected microwaves, the best-known examples of which are pulsed radar and frequency modulated continuous wave radar (FMCW radar). In pulsed radar short microwave transmission pulses are emitted periodically which are reflected by the object being measured and re-received as an echo signal after a transit time depending on the distance involved. The received signal amplitude versus time function constitutes the echo function. Every value of this echo function corresponds to the amplitude of an echo due to reflection at a certain distance from the antenna. In FMCW radar a continuous microwave is emitted which is periodically linearly frequency-modulated, for example, according to a saw-tooth function. The frequency of each received echo signal thus exhibits with respect to the instantaneous frequency which the transmission signal has at the time of reception, a difference in frequency which depends on the transit time of the echo signal. The difference between the frequency of the transmission signal and that of the reception signal which may be obtained by mixing both signals and evaluating the Fourier spectrum of the mixed signal, thus corresponds to the distance of the reflecting surface of the object away from the antenna, and the level of the frequency characteristic corresponds to the magnitude of the echo amplitude. This Fourier spectrum, therefore, constitutes the echo function in this case. In prior art the entirety of the echo function obtained in the course of a measurement cycle is stored and then the stored echo function is evaluated according to a given program by a computer to establish the useful echo and to determine the transit time of the useful echo. Storing the echo function in this case is preferably done in digitized form by the echo function being periodically sampled, each sampled value being converted by an analog-to-digital converter into a digital code group and the digital code groups being written into a RAM of the computer.
A particular problem in connection with level measurement using microwaves is that a deposit of material may build up on the antenna. This danger is more particularly to be found in the case of dusty or powdery material, especially if the antenna is moist, and furthermore in the case of sticky, viscous, spattering or stirred materials. While microwave antennas tolerate a certain degree of fouling, they cease to be operational if the deposit layer is excessive. In this case the radar signal will be completely absorbed at the antenna so that no useful echo may be detected. Using conventional methods it is accordingly no longer possible to distinguish whether there is no reflecting surface present in the beam path, whether there is heavy attenuation in the beam path clear of the antenna (for instance in the case of foam on the material), whether the antenna has been blocked by the formation of a deposit or whether material is located in the immediate vicinity of the antenna.
The object of the invention is to provide a method of the aforementioned kind which permits the recognition of the formation of deposits on the antenna and of other trouble such as, for instance, damage to or loss of the antenna and by means of which a distinction is permitted between a deposit on the antenna and material in the immediate vicinity of the antenna.